Voters look set to deal a crushing blow to Nick Clegg by rejecting the alternative vote by a two to one majority, an end of campaign Guardian/ICM poll shows.

The survey predicts a 68% no vote against just 32% for yes and, in line with other recent polls, suggests support for electoral change has slumped further since a Guardian/ICM poll last month revealed the growing size of the no lead. The lead then was 16 points, compared with 36 now.

That suggests the campaign has been overwhelmingly lost by the Yes to AV alliance, which began the year with an apparent lead in the polls. In February, a Guardian/ICM poll put the yes camp ahead by two points.

The latest poll, carried out by telephone and including a representative sample of voters in Northern Ireland, finds opposition to AV has grown among most types of voters.

A majority of Labour supporters now say they will vote against AV despite Ed Miliband's endorsement of change. So will almost nine in 10 Conservatives and more than a quarter of definite Lib Dem voters, whose lukewarm support seems confirmed by the fact that they are now less likely than Conservative or Labour voters to say they will turn out.

Young voters remain in favour of AV, but are the least likely to be certain to vote.

Among all voters, before adjustment for likely turnout, 28% said they were for change, with 52% against and 20% who said they didn't know.

After adjustment, among voters who say they are likely to vote, 32% back yes, down from 42% in April. Meanwhile 68% say no – up from 58%.

The poll was carried out over Monday and Tuesday this week. The bank holiday made it more difficult to reach some voters, but figures for the state of the parties have been adjusted to take account of any imbalance in the sample.

In a hypothetical immediate general election, Labour has 37%, unchanged from April. The Conservatives are on 36%, up one. The Liberal Democrats are on 15%, unchanged, and others on a combined 11%, down two.

•ICM Research interviewed a random sample of 1035 adults aged 18+ by telephone on 2-3rd May 2011. Interviews were conducted across the country and the results have been weighted to the profile of all adults.